Rui N. Correia

Effects of Si speciation on the in vitro bioactivity of glasses

The surface reactivity of glasses belonging to the (mol%) 31SiO2-11P2O5-(58-x)CaO-xMgO series, with x ranging from 0 to 32, was studied in Kokubos simulated body fluid (SBF). Scanning electron microscopy, inductively coupled plasma spectroscopy and Fourier transform infrared spectroscopy were used to characterise the glass surface and the SBF compositional changes. All glasses develop surface layers rich in silica and calcium phosphate. An increasing surface activity with increasing MgO/CaO ratio was observed. In a previous investigation using magic-angle spinning nuclear magnetic resonance it was found that there is an increasing abundance of Q0 species in the glass structure with increasing MgO content. The present work shows that, when immersed in SBF, Q0-rich glasses are easily leached to form a silica gel layer. It is concluded that MgO in the glass indirectly improves the early stages of mineralisation by favouring Q0 speciation. This mechanism plays an important role in glass bioactivity.

TREATMENTS TO INDUCE THE NUCLEATION AND GROWTH OF APATITE-LIKE LAYERS ON POLYMERIC SURFACES AND FOAMS

In this work, a bioactive glass is used as a percusor of calcium-phosphate (Ca-P) film deposition onto several polymer-based materials. Both bioinert (high molecular weight polyethylene, HMWPE), and biodegradable (corn starch-based blends, SEVA-C) polymers, unreinforced or reinforced with hydroxylapatite (HA), were coated by the very simple proposed route. Also polyurethane (PU) foams, with an open-cell structure, were mineralized by the proposed method. In fact, it was possible to induce the growth of the Ca-P films not only at the surface, but also in the bulk of the PU foam. These cellular materials are intended for cancellous bone replacement applications. The morphology of the formed films was strongly dependent on the used substrate, its polar character, and on the presence of HA in its composition, as observed by SEM. Nevertheless, a well defined needly like structure was observed in all samples at high magnifications. The Ca:P ratios of the films were between 1.5 and 1.7, i.e. in the range of tricalcium phosphate-hydroxylapatite. Raman spectroscopy and thin-film x-ray diffraction (XRD) evidenced the formation of mostly amorphous calcium-phosphate films. After scraping the coating from the polymer surface and heat-treating the resulting powder at 1000°C for 1 h, HA and β-tricalcium phosphate (TCP) typical peaks were found on XRD patterns.

Surface modifications of a glass and a glass-ceramic of the MgO-3CaO · P2O5-SiO2 system in a simulated body fluid

A glass of nominal composition (wt%) 17.25 MgO-52.75 3CaO.P2O5-30 SiO2 and a glass-ceramic obtained from it showed surface modifications when immersed in an acellular medium having a composition similar to that of human blood plasma. A (Ca, P)-rich layer, with an approximate Ca/P atomic ratio of 1.7, identified as hydroxyapatite, developed on both samples. The precipitated film on the glassy sample was weakly bonded, whereas that formed on the glass-ceramic was strongly adherent. The apatite precipitated during the in vitro tests on both samples grew as a needle-like structure with crystals about 150-200 nm long and 50-70 nm thick, as measured on specimens soaked for 1 month in the simulated body fluid (SBF). The presence of calcium and phosphate ions in the SBF contributed to the precipitation of the (Ca, P)-rich layers on both specimens.

WET SYNTHESIS AND CHARACTERIZATION OF MODIFIED HYDROXYAPATITE POWDERS

Hydroxyapatite powders with reproducibly different Ca/P ratios and powders with varying amounts of co-precipitated magnesium, sodium and potassium were synthesized by a wet method. Solids composition, particle size and morphology, crystalline structure, sintering behaviour and microstructure were investigated in order to understand the effect of composition in the properties of the powders. Under the present conditions of synthesis, it was concluded that magnesium, sodium and potassium will enter the hydroxyapatite lattice in vestigial amounts. Magnesium gives rise to a co-precipitated amorphous phase which crystallizes as β-whitlockite on calcining. A relationship was found between the Ca/P ratio in hydroxyapatite and its sinterability. The sintering process of pure and alkali-containing hydroxyapatites is completed at 1300 °C and leads to dense ceramics in the case of pure and sodium-containing hydroxyapatites but not in potassium-containing material.

Influence of the CaO/MgO ratio on the structure of phase-separated glasses: a solid state 29Si and 31P MAS NMR study

Abstract 29 Si and 31 P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was used to investigate structural aspects of glasses of the series (mol%) 31SiO 2 –11P 2 O 5 –(58− x )CaO– x MgO, with x ranging from 0 to 32, in which precipitation of amorphous silica occurs. As expected for alkaline-earth silicates, a strong 29 Si MAS NMR peak was recorded, shifting to higher frequencies with decreasing CaO/MgO molar ratios. Q 2 species predominate in the alkaline-earth rich phase and the observed shift in the 29 Si spectra is attributed to a decrease in the average degree of Si polymerization in the matrix. The experimental evidence suggests that Mg 2+ replaces Ca 2+ and also promotes the disruption of Si–O–Si bonds. Substitution of CaO by MgO increases the amount of dispersed phase. 29 Si MAS NMR showed the presence of Q 4 species in the dispersed phase and also suggested the presence of Q 0 species. A disproportionation reaction 2Q 2 ⇌Q 0 +Q 4 is proposed to balance the silicate charges. 31 P MAS NMR showed that phosphorus is present in an orthophosphate environment. The shift of the 31 P MAS NMR peak was attributed to the replacement of Ca by Mg.

The fluorapatite–anorthite system in biomedicine

Glasses and glass ceramics of fluorapatite-anorthite (eutectic composition) were produced and characterized in order to evaluate their potential application in biomedicine. Bio-reactivity was determined by in vitro tests by immersion of powders in simulated plasma liquids as well as by in vivo experiments by implantation in rabbits. According to the results, the investigated materials are bio-acceptable since no toxic or other harmful evidence was detected. Glass-ceramics showed remarkable inertness, whereas glasses spontaneously dissolved in SBF and after 1 week moderate formation of apatite was observed, that however ceased within a month.

Deposition of bioactive glass-ceramic thin-films by RF magnetron sputtering

Abstract Thin films of bioactive glass-ceramics have been deposited on titanium and silicon substrates by RF magnetron sputtering. The crystalline phases and the microstructure of the films have been characterized using XRD and SEM analysis; the main phases present were calcium–magnesium phosphates, enstatite and forsterite. The adhesion of the films on titanium has been examined by pull-off testing; the adhesion strength for as-deposited films was around 40 MPa, but after crystallization the strength dropped to about half this value due to the presence of cracks. Samples kept in simulated body fluid showed an apatite-like layer, suggesting that the films are bioactive.

Inorganic plasma with physiological CO2/HCO3 buffer

The substitution of tris/HCl buffer by CO(2)/HCO(3)(-) buffer in inorganic plasma was studied. An appropriate gas mixture of CO(2)/N(2) was continuously bubbled in Kokubos SBF solution prepared without addition of Tris/HCl. This method enables buffering the solution within the 7.3-7.4 pH interval and, at the same time, reaching a HCO(3)(-) concentration between 24 and 27 mmol dm(-3), which are the normal concentrations reported for blood plasma. Mineralisation studies on calcium phosphate ceramics using this solution showed that, in the presence of such hydrogencarbonate concentrations, the formation of a mineralisation layer on the ceramic occurs via a carbonated octacalcium phosphate, that evolves to carbonated hydroxyapatite. The results suggest that mineralisation studies in this new carbonate-containing simulated inorganic plasma mimic biomineralisation more closely than traditional SBF.

Mineralisation of two phosphate ceramics in HBSS: role of albumin

The role of albumin in the mineralisation process of commercial hydroxyapatite (HAp) and synthesised biphasic (HAp-tricalcium phosphate) ceramics in a bufferless simulated inorganic plasma (HBSS) was investigated by conventional in vitro tests and static and dynamic wettability measurements. Albumin was either pre-adsorbed or solubilised in HBSS. It was found that calcium complexation by albumin plays a key role in early mineralisation kinetics, so that mineralisation is favoured when albumin is pre-adsorbed and hindered when it is dissolved in HBSS. In the biphasic ceramic this picture is complicated by the fact that albumin, in solution, seems to promote the dissolution of tricalcium phosphate, and simultaneously compete for calcium with the ceramic. It also appears that albumin has a stabilising effect of octacalcium phosphate present in deposits on commercial HAp. The same effect may be present in the case of the biphasic ceramic, at earlier mineralisation times, when octacalcium phosphate appears as a precursor of HAp. Octacalcium phosphate formation on commercial apatite is accompanied by carbonate substitution in phosphate positions.

New PMMA-co-EHA glass-filled composites for biomedical applications: Mechanical properties and bioactivity

A bioactive glass of the 3CaO.P(2)O(5)-MgO-SiO(2) system was incorporated as a filler into poly(methylmethacrylate)-co-(ethylhexylacrylate) (PMMA-co-EHA) copolymer. The effect of filler proportion (0, 30, 40 and 50wt.%) on the bending properties was evaluated and a maximum flexural strength of 29MPa coupled with an elastic modulus of 1.1GPa was obtained at an intermediate filler concentration (30wt.%). These values are slightly higher than those usually reported for human cancellous bone. The in vitro bioactivity was assessed by determining the changes in surface morphology and composition after soaking in simulated body fluid (SBF, Kokubo solution). Inductively coupled plasma was used to trace the evolution of ionic concentrations in the SBF solution, namely Ca and P. X-ray diffraction and scanning electron microscopy confirmed the growth of spherical calcium phosphate aggregates on the surface of composites, indicating that the composites are potentially bioactive.